1. Field of the Invention
The present invention is directed to a method and an apparatus for non-invasive, in vivo determination of blood constituents of the type wherein light absorption in a body part is monitored while the body part is being subjected to an external mechanical influence.
2. Description of the Prior Art
A method and an apparatus of the above general type for determining a blood constituent are disclosed by U.S. Pat. No. 5,372,135. Blood is thereby expressed by external pressure pulses from the tissue to be examined in order to obtain spectra given different blood volumes. Measured values with and without external pressure are subtracted from one another and difference spectra are thereby acquired. Light wavelengths varied by an acousto-optical filter are thereby utilized. The concentration of the blood constituent, specifically of blood glucose, is then determined from the difference spectra. In U.S. Pat. No. 5,372,136, the modification of the transirradiated light intensity by pulsating blood (AC value) and the transirradiated light intensity itself (DC value) are evaluated at two wavelengths at which the constituent (hematocrit) to be identified respectively absorbs. In addition, the absorption of the constituents (water) not to be identified is at least ten times lower then the absorption of the constituent (hematocrit) to be identified. In particular, these wavelengths are isobestial, i.e. the absorption coefficient of oxygenated and de-oxygenated hemoglobin is the same. Both the natural blood pulse as well as an artificial pulsation with the assistance of a stepping motor can be utilized for the AC values. The possibility of utilizing the method for the identification of other blood constituents is referenced without more detailed particulars.
Measuring blood constituents, particularly glucose, non-invasively by the measurement of the absorption of light is known from the references of E. Stohr et al, "Quantitative FT-IR Spectometry of Blood Constituents", conference proceedings 14th Annual International Conference of the IEEE-EMBS, Paris, Oct. 29, through Nov. 1, 1957 and H. M. Heise, "Technology for Non-Invasive Monitoring of Glucose", Conference Proceedings 18th Annual Conference of the IEEE-EMBS, Oct. 31, through Nov. 3, 1996, Amsterdam.
When measuring the concentration of constituents, it is often a complicating factor that the measured quantity is also sensitively dependent on other parameters then the concentration of a constituent. No reproducible signal can then be obtained without constant re-calibration.
This problem particularly occurs when the concentration of blood sugar is to be identified non-invasively in vivo. Specific optical measuring methods are conventionally employed for this purpose, such as rotation of the polarization plane dependent on the concentration, optical or acousto-optical spectroscopy of the infrared bands of the sugar, techniques making use of the Raman effect and techniques making use of light scatter in the tissue which changes with the glucose concentration.
The determination of the glucose concentration by optical spectroscopy is complicated by the superimposition of the absorption bands of water. Attempts are therefore often made to measure the glucose concentration at wavelength pairs that are sought out such that only water absorbs at the one wavelength but water and glucose absorb at the other. True-to-scale subtraction of the absorption signals then yields a signal value proportional to the glucose concentration.
A problem with this known method, however, is that even extremely minute fluctuations of the scaling of factor lead to errors that cannot be compensated.
The idea of initially making the beam path empty of blood with a pressure capsule in order to obtain a defined initial measured value and to then obtaining a further measured value with the blood flowing back in derives from Wood and Geraci (1949). This principle was employed for the optical identification of the blood oxygenation (E. H. Wood and J. E. Geraci, Photoelectric determination of arterial oxygen saturation in man, Journ. Lab. Clin. Med. 34, 387-401 (1949)).
The article by L. A. Geddes, "Heritage of the Tissue-Bed Oximeter", which appeared in IEEE Engineering in Medicine and Biology, March/April 1997, pp. 87-91, provides an overview of various embodiments of measuring devices for the non-evasive determination of blood oxygen concentration.
East German Patent 107 982 discloses a method and an apparatus for the analysis of emitted radiation of pressure-modulated gases for concentration identification. The concentration-dependent emission of light thereby ensues from a flow-through bulb. In the context of this East German patent, emitted radiation is the cell-luminescence of the gas that, for example, is excited by a gas discharge.
British Specification 2 262 337 A is likewise directed to the spectroscopy of gases, whereby the absorption of a reference cell is pressure-modulated with an acoustic resonator.
U.S. Pat. No. 5,539,207 discloses a technique for identifying tissue by infrared spectroscopy with and without pressure by comparing to spectra of a known tissue. No constituents are quantified.
For determining blood constituents, the aforementioned U.S. Pat. No. 5,372,136 discloses that tissue be transirradiated with light of a plurality of wavelengths and that the pulsing blood stream be simulated by a compress operated with a stepping motor.
PCT Application WO 98/43096 discloses how blood glucose can be identified by application of light of a plurality of wavelengths and active induction of a harmonic change of the blood volume, for example with an inflatable balloon or by temperature variation.